eel

Introduction
General features
Natural history
Form and function
Evolution, paleontology, and classification
- Distinguishing taxonomic features
- Annotated classification
- Critical appraisal

References & Edit History Quick Facts & Related Topics

Images

Illustration showing adult and larval forms of various eel species, including Nemichthys, Cyema, Anguilla, Gymnothorax, and Conger, with adults on the left and their transparent, leaf-shaped larvae on the right.

Illustration showing the metamorphosis of the American eel (Anguilla rostrata), with seven stages labeled A to G. The top three show increasingly larger transparent larvae, followed by three transitional forms, and ending with a more developed, elongated glass eel.

A Conger oceanicus eel with a long, smooth, dark body and open mouth emerges from a rocky crevice surrounded by colorful coral.

At a Glance

eel summary

Quizzes

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Related Questions

How can electric eels force hidden prey to reveal its location?
How do conger eels differ from moray eels?
How do electric eels defend themselves by leaping from the water?
How do electric eels use low-voltage electrical pulses in murky water?
How do garden eels live in large colonies on tropical reefs?
How do moray eels capture their prey?
How do ocean currents spread eel larvae far from spawning areas?
How do snake eels burrow into the seafloor?
How do swamp eels differ from true eels?
How do true eels differ from electric eels?
How does an eel larva differ in appearance from an adult eel?
How does an electric eel use its mouth to take in oxygen?
How does the continuous fin around an eel’s tail help it swim?
How is sex determined in European freshwater eels?
How long can eel larvae remain in the open ocean?
How many species are included in the order Anguilliformes?
What are the three electric organs in an electric eel’s body?
What changes occur when a leptocephalus transforms into an elver?
What do freshwater eels eat as juveniles and adults?
What happens to European freshwater eels after they spawn?
When did eels first appear in Earth’s history?
Where do moray eels usually hide during the day?
Where do most eel species live?
Which animals prey on leptocephalus larvae?
Which eel species can grow to about 3.5 metres long?
Why are young freshwater eels called glass eels?
Why can a moray eel bite cause a serious wound?
Why do mature freshwater eels migrate from rivers to the ocean?
Why must electric eels regularly surface to gulp air?
What is a ray?
How do rays differ from sharks?
What makes the tails of many stingrays dangerous?
What are the physical characteristics of sharks?
What is shark finning and why is it controversial?
How do sharks reproduce?
What is the main characteristics that define vertebrates?
How are vertebrates classified in terms of their morphology?
How do amphibians differ from reptiles in terms of reproduction?
How do fish sleep?

Science Birds, Reptiles & Other Vertebrates Fish

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External Websites

The Royal Society Publishing - Biology Letters - Deep-ocean origin of the freshwater eels
Nagao Natural Environment Foundation - Anguilliformes
Department of Conservation - Eel
European Commission - Eel
National Center for Biotechnology Information - PubMed Central - Genomics in Eels Â— Towards Aquaculture and Biology
Animal Corner - Galapagos Eels
PBS - Nature - Eels
Food and Agriculture of the United Nations - Order Anguilliformes Anguillidae (PDF)
Smithsonian Ocean - The Origin of Eels

Britannica Websites

Articles from Britannica Encyclopedias for elementary and high school students.

eel - Children's Encyclopedia (Ages 8-11)
eel - Student Encyclopedia (Ages 11 and up)

eel

fish

Also known as: Anguilliformes, anguilliform

Written by

Peter Henry John Castle

Reader in Zoology, Victoria University of Wellington, New Zealand. Author of numerous papers on eels.

Peter Henry John Castle

Fact-checked by

Britannica Editors

Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree....

Britannica Editors

Last updated

Oct. 23, 2026 •History

eel, (order Anguilliformes), any of more than 800 species of teleost fishes characterized by elongate wormlike bodies. Anguilliforms include the common freshwater eels as well as the voracious marine morays. Regardless of their final habitat, all eels probably pass through the leptocephalus stage, an extended larval phase, in the open ocean and undergo metamorphosis to a juvenile stage that is a smaller version of the adult. At maturity eels range in length from 10 cm...

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